I think I just became a QBist ?

[atyy]

Smerlak and Rovelli criticize the notion of a classical observer. Is it fair, however, to say that RQM assumes a classical spacetime?

 

[strangerep]

Smerlak and Rovelli criticize the notion of a classical observer. Is it fair, however, to say that RQM assumes a classical spacetime?

This question (to which I can't offer a clear answer) nevertheless made me remember a few other papers which may be pieces of the puzzle.

They've been discussed here on BTSM in the past, but here are the main references...

----------------------------
S. Gielen, D. Wise, "Lifting General Relativity to Observer Space",
J. Math. Phys. 54, 052501 (2013), http://arxiv.org/abs/1210.0019

Abstract:
The `observer space' of a Lorentzian spacetime is the space of future-timelike unit tangent vectors. Using Cartan geometry, we first study the structure a given spacetime induces on its observer space, then use this to define abstract observer space geometries for which no underlying spacetime is assumed. We propose taking observer space as fundamental in general relativity, and prove integrability conditions under which spacetime can be reconstructed as a quotient of observer space. Additional field equations on observer space then descend to Einstein's equations on the reconstructed spacetime. We also consider the case where no such reconstruction is possible, and spacetime becomes an observer-dependent, relative concept. Finally, we discuss applications of observer space, including a geometric link between covariant and canonical approaches to gravity.
------------------------
(My emboldening.)

(See also the references therein to their earlier work. The basic idea is to start from a nonholonomic field of observers (meaning a nonintegrable field of tetrad reference frames, iiuc).

Gielen & Wise cite the following paper (also discussed here before, iirc):

G. Amelino-Camelia, L.tFreidel, J. Kowalski-Glikman, L. Smolin,
"The principle of relative locality",
http://arxiv.org/abs/1101.0931

Abstract:
We propose a deepening of the relativity principle according to which the invariant arena for non-quantum physics is a phase space rather than spacetime. Descriptions of particles propagating and interacting in spacetimes are constructed by observers, but different observers, separated from each other by translations, construct different spacetime projections from the invariant phase space. Nonetheless, all observers agree that interactions are local in the spacetime coordinates constructed by observers local to them.
This framework, in which absolute locality is replaced by relative locality, results from deforming momentum space, just as the passage from absolute to relative simultaneity results from deforming the linear addition of velocities. Different aspects of momentum space geometry, such as its curvature, torsion and non-metricity, are reflected in different kinds of deformations of the energy-momentum conservation laws. These are in principle all measurable by appropriate experiments. We also discuss a natural set of physical hypotheses which singles out the cases of momentum space with a metric compatible connection and constant curvature.

 

[strangerep]

Looks like N.D.Mermin is still thinking about this stuff...

N. D. Mermin,
QBism as CBism: Solving the Problem of "the Now",
Available as: http://arxiv.org/abs/1312.7825

Abstract:

The QBist view of science, first put forth to dispel the fog from quantum foundations, also clears up a longstanding puzzle in classical physics.

 

[RUTA]

Looks like N.D.Mermin is still thinking about this stuff...

N. D. Mermin,
QBism as CBism: Solving the Problem of "the Now",
Available as: http://arxiv.org/abs/1312.7825

Abstract:

I first heard this very same solipsist solution over 20 years ago from a philosopher of science, so I doubt it's news in that circle. While I agree with it and believe it's important to understand science as an objectification/unification of multifarious subjective experience, I would like to see how he thinks this understanding might lead to new theories of physics.

 

[atyy]

In http://arxiv.org/abs/quant-ph/0205039 it seems that Qbism does believe in reality, and that it is not a solipsist position. One motivating factor in its approach is that if we wish to constrain the possible realities underlying quantum mechanics, let us try to see how much of it we can derive by "logic". What we can't derive will then be common to all possible underlying realities. That much seems reasonable to me.

However, in my understanding, because QBism draws on the analogy between Bayesian conditioning and wave function collapse, it seems to me to lead in part to asking about possible hidden variable theories that are psi-epistemic (eg. http://arxiv.org/abs/1201.6554, http://arxiv.org/abs/1303.2834). I'm not sure that the QBists would agree though.

Interestingly, Leifer and Spekkens http://arxiv.org/abs/1107.5849 comment "In this sense, our approach is more closely aligned in its philosophical starting point with quantum Bayesianism, the view developed by Caves, Fuchs and Schack", but "Unlike the quantum Bayesians, however, we are not committed to the notion that the beliefs represented by quantum states concern the outcomes of future experiments. Rather, the picture we have in mind is of the quantum state for a region representing beliefs about the physical state of the region, even though we do not yet have a model to propose for the underlying physical states."

 

[RUTA]

In http://arxiv.org/abs/quant-ph/0205039 it seems that Qbism does believe in reality, and that it is not a solipsist position. One motivating factor in its approach is that if we wish to constrain the possible realities underlying quantum mechanics, let us try to see how much of it we can derive by "logic". What we can't derive will then be common to all possible underlying realities. That much seems reasonable to me.

However, in my understanding, because QBism draws on the analogy between Bayesian conditioning and wave function collapse, it seems to me to leads in part to asking about possible hidden variable theories that are psi-epistemic (eg. http://arxiv.org/abs/1201.6554, http://arxiv.org/abs/1303.2834). I'm not sure that the QBists would agree though.

I call this view "solipsistic" because their reality isn't the unified (objective) reality represented by the spacetime manifold of physics, but the disjoint collection of subjective experience. Sorry if I've misused the term. Here is what I'm referring to (p 4):
*****************************************************
To represent my actual experiences as a collection of mathematical points in a continuous space-time is a brilliant strategic simplification, but we ought not to confuse a cartoon that concisely attempts to represent our experience, with the experience itself.

IV. The Now of one person
If I take my experience of Now as the reality it clearly is to me, and recognize that space-time is an abstract diagram that I use to organize such experiences, then the place of the Now in physics becomes obvious. At any moment I can picture my past experience in my diagram as a continuous time-like curve that terminates in my Now.
*****************************************************
So, reality is to be associated with individual experience rather than its unified (from the experiences of all individuals) depiction/"cartoon" (objectification).

Anyway, have you seen any proposed modifications to physics based on this view?

 

[atyy]

@RUTA, yes, the approach does seem solipsistic to me in some presentations. However, they say it isn't, and I've been trying to understand it from their point of view.

I haven't seen any proposed modification to physics based on this view. The closest I've seen is maybe Oreshkov, Costa and Brukner's http://arxiv.org/abs/1105.4464 , because I consider QBism and Leifer and Spekkens http://arxiv.org/abs/1107.5849v4 work to be related, and Leifer and Spekkens mention Oreshkov et al. But perhaps that is too much of a stretch.

 

[RUTA]

@RUTA, yes, the approach does seem solipsistic to me in some presentations. However, they say it isn't, and I've been trying to understand it from their point of view.

I haven't seen any proposed modification to physics based on this view. The closest I've seen is maybe Oreshkov, Costa and Brukner's http://arxiv.org/abs/1105.4464 , because I consider QBism and Leifer and Spekkens http://arxiv.org/abs/1107.5849v4 work to be related, and Leifer and Spekkens mention Oreshkov et al. But perhaps that is too much of a stretch.

Thanks for the reference, atyy. I don't believe it's a stretch at all to relate it to Mermin's perspective.

 

[marcus]

I think you misinterpreted what D.M. is saying. Reality is definitely out there: we all have our individual experiences of it and happily we can AGREE and harmonize our findings (transform from one observer to another.)
Reality is what we experience and see around us and make measurements of and WANT TO UNDERSTAND. We invent math metaphors to organize our accumulated knowledge and to predict about future knowledge.
Basically he says don't confuse the metaphor with the reality.

In reality there is no 4D diffy manifold and "events" are not points in that 4D filing device. That's just an convenient device to organize approximate relationships amongst bits of knowledge. That also helps us translate one guy's experience over into another guy's. The math metaphor is good for organizing and translating.

I call this view "solipsistic" because their reality isn't the unified (objective) reality represented by the spacetime manifold of physics, but the disjoint collection of subjective experience. Sorry if I've misused the term. Here is what I'm referring to (p 4):
*****************************************************
To represent my actual experiences as a collection of mathematical points in a continuous space-time is a brilliant strategic simplification, but we ought not to confuse a cartoon that concisely attempts to represent our experience, with the experience itself.

IV. The Now of one person
If I take my experience of Now as the reality it clearly is to me, and recognize that space-time is an abstract diagram that I use to organize such experiences, then the place of the Now in physics becomes obvious. At any moment I can picture my past experience in my diagram as a continuous time-like curve that terminates in my Now.
*****************************************************
So, reality is to be associated with individual experience rather than its unified (from the experiences of all individuals) depiction/"cartoon" (objectification).

Anyway, have you seen any proposed modifications to physics based on this view?

If you go back to where he says what you quoted it is actually not at all Solipsy but very COMMON SENSE. He says commonly accepted 4D continuum physics has no place for NOW. Everybody has their world-line but physics does not give us a glowing dot that is special on our world line. But, he says, we all experience NOW. So that is a problem (which has bothered various smart people.)
That was because the smart people took the 4D continuum with the worldlines in it TOO SERIOUSLY. Each person's NOW is a real part of reality, that is common sense. The 4D continuum that has no place in it for that facet of reality is just a handy metaphorical device. There is no reason it should be COMPLETE it just has to be useful. So, no problem.

 

[atyy]

Frankly, I don't believe in Mermin's solution. The "now" of perception is almost certainly not the now of a point in a spacetime diagram. The now in a spacetime diagram is a point of infinite resolution. The now of a person is blurred over several milliseconds, and is placed by us in a constructed narrative involving memory. I believe Rovelli was much closer to the mark when he distinguished in his book different concepts of time, including thermodynamic time and psychological time.

 

[RUTA]

I think you misinterpreted what D.M. is saying. Reality is definitely out there: we all have our individual experiences of it and happily we can AGREE and harmonize our findings (transform from one observer to another.)
Reality is what we experience and see around us and make measurements of and WANT TO UNDERSTAND. We invent math metaphors to organize our accumulated knowledge and to predict about future knowledge.
Basically he says don't confuse the metaphor with the reality.

In reality there is no 4D diffy manifold and "events" are not points in that 4D filing device. That's just an convenient device to organize approximate relationships amongst bits of knowledge. That also helps us translate one guy's experience over into another guy's. The math metaphor is good for organizing and translating.



If you go back to where he says what you quoted it is actually not at all Solipsy but very COMMON SENSE. He says commonly accepted 4D continuum physics has no place for NOW. Everybody has their world-line but physics does not give us a glowing dot that is special on our world line. But, he says, we all experience NOW. So that is a problem (which has bothered various smart people.)
That was because the smart people took the 4D continuum with the worldlines in it TOO SERIOUSLY. Each person's NOW is a real part of reality, that is common sense. The 4D continuum that has no place in it for that facet of reality is just a handy metaphorical device. There is no reason it should be COMPLETE it just has to be useful. So, no problem.

Hi marcus! Always enjoy your input

I sent the paper to a colleague in philosophy of science who has coauthored papers with me in foundations and he also called it a solipsistic view. Again, it's semantics, but it's in his discipline.

Regardless of what we choose to call it, I think it can be useful for us physicists to consider the consequences of the view (e.g., paper atyy referenced). Even if we disagree on what D.M. means by what he writes, the discussion itself explores possible ways to rethink physics and can be useful as physics in that sense.

Therefore, let me repeat this part of the D.M. quote:
If I take my experience of Now as the reality it clearly is to me, and recognize that space-time is an abstract diagram that I use to organize such experiences
Notice that "reality" is being equated with "experience," not "that which is being experienced." And, spacetime is characterized as an organizational device. Thus, physics is understood to unify the disparate experiences of individuals (at least concerning phenomena where that is possible). The model that physics creates to unify these experiences is a 4D manifold with no preferred Now (you could say every point on every worldline is a Now, but none of them "glows"). The problem of the Now happens when one promotes the cartoon to reality (reifies spacetime) and reality loses its "glow." The "glow" = "my experience of the Now" is part of "the reality" so your reified spacetime is missing some "reality."

Ok, so how does that help us rethink the way we do physics? See the paper posted by atyy whereby "space-time may emerge from a more fundamental structure in a quantum-to-classical transition."

 

[marcus]

Frankly, I don't believe in Mermin's solution..

?
But what you say is exactly what Mermin was saying in what I just read.

The "now" of perception is almost certainly not the now of a point in a spacetime diagram. The now in a spacetime diagram is a point of infinite resolution. The now of a person is blurred over several milliseconds, and is placed by us in a constructed narrative involving memory.

IIRC it was in his annotated interview. He pointed out the artificiality of the "point in a spacetime diagram" the unrealistic "infinite resolution".

So what you say makes me think that you actually might agree with Mermin's position but just not realize that you do. It strikes me as commonsensical and realistic. He does not say that David Mermin's perception is THE ONE REALITY (that would be Solipsy) he just observes that each of our individual experience of NOW is undeniably real, it's a part of life.

Like Aristotle said, reality is what we all see (experience) and can agree about---and (I can add) want to understand.
We don't have to have a single verbal or math metaphor for it that exemplifies all the observed correlations so that we can pretend that one single metaphor "really IS" physical reality. Just be content and confident that there is a reality, which we want to know about and describe its correlations (so we can make rational bets on future, for one thing) and which we are gradually getting to understand better.

 

[atyy]

So what you say makes me think that you actually might agree with Mermin's position but just not realize that you do. It strikes me as commonsensical and realistic. He does not say that David Mermin's perception is THE ONE REALITY (that would be Solipsy) he just observes that our individual experience of NOW is undeniably real, its a part of life.

Honestly, that's my major problem with Mermin's position. If what he is saying is just common sense (and it seems to be), why does he write it as if it's such a big deal? Isn't he just saying "all models are wrong, but some are useful"? My common sense tells me he's the last person to discover it. But since I want to give him some credit, I assume he what he is saying is not common sense.

There is one part where I think I definitely disagree with him. Isn't the perception of "now" part of the "hard problem" of consciousness? Like is the "blue" I see the same as the "blue" you see? (Yes - see his footnote 9!)

There's Nagel's famous essay http://organizations.utep.edu/portals/1475/nagel_bat.pdf, which is why a book about the philosophy of baseball should be called "What is it like to be at bat?"

 

[marcus]

Notice that "reality" is being equated with "experience," not "that which is being experienced."

I think you are over-interpreting. He didn't equate his experience with the sole reality. He just said that his experience is real. It is a part of reality, if you will. It has realness. Who can argue with that. We all experience and experience a present moment, or so I trust….everybody I've encountered seems similar in that regard.

And, spacetime is characterized as an organizational device. Thus, physics is understood to unify the disparate experiences of individuals (at least concerning phenomena where that is possible). The model that physics creates to unify these experiences is a 4D manifold with no preferred Now (you could say every point on every worldline is a Now, but none of them "glows"). The problem of the Now happens when one promotes the cartoon to reality (reifies spacetime) and reality loses its "glow." The "glow" = "my experience of the Now" is part of "the reality" so your reified spacetime is missing some "reality."

Yes! You put it as clearly as Mermin did, maybe more concisely too.

Ok, so how does that help us rethink the way we do physics? See the paper posted by atyy whereby "space-time may emerge from a more fundamental structure in a quantum-to-classical transition."

I'll have a look. Here are some links that Atyy posted recently in this thread (
http://arxiv.org/abs/1105.4464
http://arxiv.org/abs/1107.5849v4
http://arxiv.org/abs/quant-ph/0205039
http://arxiv.org/abs/1201.6554,
http://arxiv.org/abs/1303.2834
Not sure which is the one you are referring to.

Right off the bat I wouldn't expect it to help us rethink physics, except pedagogically. It might change how profs explain to students. Mermin compares his QB interpretation with what he thinks about "consistent histories" and Bohmery and he thinks about teaching students about quantum computers and quantum computation. He says his interpretive framework is better adapted to the teaching job. He says Bohm or consistories would be a clumsy unmanageable framework to calculate in and explain and teach in. Maybe he is right. I can't say about that.
YOU might have an interpretation of QM that works better. Again I don't know.

 

[atyy]

I'm not sure why RUTA agreed the Oreskov, Costa & Brukner paper http://arxiv.org/abs/1105.4464 was somehow linked to QBism. But let me give a guess. In the textbook (eg. Landau & Lifshitz) version of quantum mechanics, to do QM we always have to divide the universe into classical and quantum bits. There is not necesssarily a wave function of the universe. The textbook version is operational, and agnostic to whether there is no further reality (some versions of Copenhagen), or that there is a deeper reality (Bohmian, many-worlds). In classical general relativity, gravity is a theory of the whole universe. So if quantum gravity is a theory of the whole universe, then we cannot use the textbook interpretation. However, perhaps quantum gravity can emerge from a theory where we still have a Heisenberg or von Neumann cut? The paper by Oreskov, Costa & Brukner assumes such a cut. I believe Oeckl's general boundary formalism also assumes such a cut, eg. http://arxiv.org/abs/gr-qc/0312081: "It is not necessary to endorse "realist" interpretations of quantum mechanics for this formulation to make sense. This is in contrast to other approaches which presume for example the existence of a "wave function of the universe"."

He gives more motivation for this in http://arxiv.org/abs/hep-th/0612076:
"In quantum gravity there is no metric background to separate systems. What is worse, diffeomorphism gauge symmetry makes any kind of (even relative) localization difficult. In particular, there is no causality or cluster decomposition property from the outset. Hence, at least a priori we cannot avoid that states are now really states of the whole universe. Apart from technical problems this also prompts deep conceptual problems such as to the meaning of quantum theory without an outside observer. It seems we have to do quantum cosmology.

While it might very well be that the mentioned problem can be solved within the standard formulation of quantum theory, it can be avoided in the general boundary formulation. State spaces, amplitudes and probabilities referring to local regions of spacetime allow to describe their physics independent of the physics outside. In particular, there is now no difficulty in placing the observer outside of the quantum mechanical process under consideration. Indeed, this suggests that we should only allow local regions in a quantum theory of gravity. Infinitely extended regions or regions “wrapping around the universe” would not be admissible."

I think Witten discuessed similar difficulties in http://arxiv.org/abs/hep-th/0106109:
"We discuss the difficulties in defining any precisely calculable or measurable observables in an asymptotically de Sitter spacetime, and explore some meta-observables that appear to make mathematical sense but cannot be measured by an observer who lives in the spacetime. ...

If quantities with this degree of precision do not exist – which seems to be the case in de Sitter space if one rejects the meta-observables – then it is not clear just what one should aim to compute. This question has nothing specifically to do with string theory, and any answer to it that makes sense might make sense in string theory."

 

[strangerep]

[...] If what [Mermin] is saying is just common sense (and it seems to be), why does he write it as if it's such a big deal?

Maybe because the notion of a physically-real global spacetime manifold is so deeply entrenched, yet easily questioned by pointing out that each of us only synthesize it based on received stimuli.

There is one part where I think I definitely disagree with him. Isn't the perception of "now" part of the "hard problem" of consciousness? Like is the "blue" I see the same as the "blue" you see?

It doesn't need to be "the same". The description of it only needs to be covariant (in a generalized sense -- my frequency perception spectrum must be consistently calibratable against yours, so that we can agree whether something is/isn't "blue").

 

[strangerep]

[...] so how does that help us rethink the way we do physics?

For me, it helps place more emphasis on time evolution as a semigroup, not a group. We do not "remember" the future, but drawing a spacetime diagram makes it too easy to imagine moving backwards and forwards in time,and less easy to see a semigroup (no inverses) structure. In modern mainstream physics, causality is merely added in by hand as an extra constraint on top of such manifold and group structures.

Those papers of Gielen & Wise I cited earlier, dealing with nonholonomic sets of observers, (actually their local tetrads), may be related to all this.

 

[atyy]

It doesn't need to be "the same". The description of it only needs to be covariant (in a generalized sense -- my frequency perception spectrum must be consistently calibratable against yours, so that we can agree whether something is/isn't "blue").

Not sure whether it works in this case, but if there are two calibrations, and they are not consistent, maybe we could test the "perception". Suppose the sensory input is 1 and 0, and I perceive it as A and B while you perceive it as B and A, and for the purpose of the logic we agree on everything, ie. we agree 1 is "A" and 0 is "B". But if there is a part of the brain that likes A and hates B, and if this part is the same in you and me, then when we see 1, I will like the input and you will hate it. If we switch the part of the brain that does the liking, then I will hate 1 and you will like it. Then we will have some evidence that we don't see 1 and 0 the same, but we both like A and hate B the same. (I heard this argument from a fellow soldier doing compulsory military service - Phillip - don't remember his last name, but credit goes to him if this argument is right.)

For me, it helps place more emphasis on time evolution as a semigroup, not a group. We do not "remember" the future, but drawing a spacetime diagram makes it too easy to imagine moving backwards and forwards in time,and less easy to see a semigroup (no inverses) structure. In modern mainstream physics, causality is merely added in by hand as an extra constraint on top of such manifold and group structures.

Those papers of Gielen & Wise I cited earlier, dealing with nonholonomic sets of observers, (actually their local tetrads), may be related to all this.

Hmmm, quite a different argument from the one I've heard. I usually think we do remember the future - it's called prediction. However, because entropy increases with time, we remember the future less than the past. So time in relativity has no arrow. The arrow belongs to thermodynamic time, which is a quirk of the initial condition.

Incidentally, I think a much more lucid essay than Mermin's was written on the same subject by this Nikolic guy Block time: Why many physicists still don’t accept it? :)

 

[Demystifier]

Incidentally, I think a much more lucid essay than Mermin's was written on the same subject by this Nikolic guy Block time: Why many physicists still don’t accept it? :)

Thanks atyy!

Since I have seen above discussions of quantum interpretations which are partially solipsistic, perhaps my paper on solipsistic hidden variables may also be relevant here:
http://lanl.arxiv.org/abs/1112.2034

 

[Demystifier]

Let me also say a few words on the Mermin's essay.

I think all this can be reduced to the following question:
Who is more clever, the physics equations, or the physicists who invented them?

If physicists are more clever, and equations merely represent a part of all things which they understand, then Mermin is right: Equations are nothing but a part of our description of our knowledge about the world, not the reality. If so, then there is no problem of now, no problem of interpretation of quantum mechanics, etc.

However, there are good reasons to believe that equations are more clever than the physicists who invented them. In other words, equations know a lot which their inventers do not. For example, Dirac new nothing about positrons when invented the Dirac equation, and the inventers of quantum electrodynamics new nothing about 10 digits of the quantity g-2.

So, as equations seem to know more than their inventors, it is hard not to take the equations seriously and believe that they represent something more than merely our current incomplete knowledge about the world. Of course, with such an attitude, there is a problem of now and there is a problem of interpretation of quantum mechanics, because the equations we currently know do not provide a direct answer.

 

[strangerep]

Not sure whether it works in this case, but if there are two calibrations, and they are not consistent, maybe we could test the "perception". Suppose the sensory input is 1 and 0, and I perceive it as A and B while you perceive it as B and A, and for the purpose of the logic we agree on everything, ie. we agree 1 is "A" and 0 is "B". But if there is a part of the brain that likes A and hates B, and if this part is the same in you and me, then when we see 1, I will like the input and you will hate it. If we switch the part of the brain that does the liking, then I will hate 1 and you will like it. Then we will have some evidence that we don't see 1 and 0 the same, but we both like A and hate B the same. [...]

I'm not really sure what your point is here.

I usually think we do remember the future - it's called prediction.

That's an... "interesting" redefinition of English words.

I would have said we "speculate" about the future (though sometimes "speculate" and "remember" get mixed up, such as when dreaming, or during a state of delirium).

However, because entropy increases with time, we remember the future less than the past.

Then why do I jump out of my skin when an unexpected loud noise occurs nearby? Entropy surely didn't increase so much that I would totally "forget" that such a significant event is about to happen in my immediate future...

 

[strangerep]

[...]
It doesn't need to be "the same". The description of it only needs to be covariant (in a generalized sense -- my frequency perception spectrum must be consistently calibratable against yours, so that we can agree whether something is/isn't "blue").

Atyy,

Your previous reply prompted a followup thought that different observers also need to share a set of observables, presumably organized as a Lie algebra, so that they are working with a common set of types. But... then there are other issues: the algebras could appear different, while nevertheless being isomorphic.

 

[Paulibus]

The description which physicists give of Nature, however mathematically sophisticated it is, includes certain elements which appear the same to all observers, namely constants like c and h.

These seem to me to be discovered aspects of nature, parts of discovered reality that emerge from the consensual but invented language of mathematics used by physicists to describe this reality with S.I. units.

As Audioloop commented in #52 --- perhaps too simplistically --- "... NATURE is more than equations".

 

[atyy]

I'm not really sure what your point is here.

Your previous reply prompted a followup thought that different observers also need to share a set of observables, presumably organized as a Lie algebra, so that they are working with a common set of types. But... then there are other issues: the algebras could appear different, while nevertheless being isomorphic.

My point is I think Mermin is using the wrong Einstein theory of relativity. He should use - not sure if this is apocryphal or not -this Einstein theory of relativity: ""When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute and it's longer than any hour. That's relativity.""

That's an... "interesting" redefinition of English words.

I would have said we "speculate" about the future (though sometimes "speculate" and "remember" get mixed up, such as when dreaming, or during a state of delirium).

Then why do I jump out of my skin when an unexpected loud noise occurs nearby? Entropy surely didn't increase so much that I would totally "forget" that such a significant event is about to happen in my immediate future...

Because thermodynamics is not the full answer. Psychological time is not the same as a thermodynamic time, although I believe the thermodynamic arrow is part of the answer for why we experience a flow of time. I like Demystifier's essay on this very much. If I understand corectly, Rovelli implicitly advocates a similar point of view in his book on quantum gravity. If this point of view is correct, then Mermin is not even understanding the question. It's like using decoherence to solve the measurement problem.

Your previous reply prompted a followup thought that different observers also need to share a set of observables, presumably organized as a Lie algebra, so that they are working with a common set of types. But... then there are other issues: the algebras could appear different, while nevertheless being isomorphic.

This is a bit tangential to the above thought, but anyway, it seems to be along "operational" lines, so I thought I'd share another BTSM paper that takes on "operational point of view", written by Schroedinger's Rat and Harald Wunderlich: http://arxiv.org/abs/0907.0372

 

[marcus]

Smerlak and Rovelli criticize the notion of a classical observer. Is it fair, however, to say that RQM assumes a classical spacetime?

This question ...made me remember a few other papers which may be pieces of the puzzle…

==quote Strangerep post #32==

They've been discussed here on BTSM in the past, but here are the main references...

----------------------------
S. Gielen, D. Wise, "Lifting General Relativity to Observer Space",
J. Math. Phys. 54, 052501 (2013), http://arxiv.org/abs/1210.0019

Abstract:
The `observer space' of a Lorentzian spacetime is the space of future-timelike unit tangent vectors. Using Cartan geometry, we first study the structure a given spacetime induces on its observer space, then use this to define abstract observer space geometries for which no underlying spacetime is assumed. We propose taking observer space as fundamental in general relativity, and prove integrability conditions under which spacetime can be reconstructed as a quotient of observer space. Additional field equations on observer space then descend to Einstein's equations on the reconstructed spacetime. We also consider the case where no such reconstruction is possible, and spacetime becomes an observer-dependent, relative concept. Finally, we discuss applications of observer space, including a geometric link between covariant and canonical approaches to gravity.
------------------------
(My emboldening.)

(See also the references therein to their earlier work. The basic idea is to start from a nonholonomic field of observers (meaning a nonintegrable field of tetrad reference frames, iiuc).

Gielen & Wise cite the following paper (also discussed here before, iirc):

G. Amelino-Camelia, L.tFreidel, J. Kowalski-Glikman, L. Smolin,
"The principle of relative locality",
http://arxiv.org/abs/1101.0931

Abstract:
We propose a deepening of the relativity principle according to which the invariant arena for non-quantum physics is a phase space rather than spacetime. Descriptions of particles propagating and interacting in spacetimes are constructed by observers, but different observers, separated from each other by translations, construct different spacetime projections from the invariant phase space. Nonetheless, all observers agree that interactions are local in the spacetime coordinates constructed by observers local to them.
This framework, in which absolute locality is replaced by relative locality, results from deforming momentum space, just as the passage from absolute to relative simultaneity results from deforming the linear addition of velocities. Different aspects of momentum space geometry, such as its curvature, torsion and non-metricity, are reflected in different kinds of deformations of the energy-momentum conservation laws. These are in principle all measurable by appropriate experiments. We also discuss a natural set of physical hypotheses which singles out the cases of momentum space with a metric compatible connection and constant curvature.
==endquote==

I think Strangerep is setting out some diverse researches that COULD be seen as signs of a novel form of REALISM or a new ontology. In this novel perspective there is a single reality which we can all see and about which we communicate and try to arrive at common understanding. But the novelty is that the various observers cannot construct a single overarching 4D continuum. There is no one spacetime that they have in common.

So Atyy's question about whether one of the approaches assumes a classical spacetime points to a key issue.

 

[Demystifier]

...but NATURE is more than equations.
.

It certainly is, but that constatation alone cannot resolve any problem one might have with the equations. Perhaps it can give someone a reason not to search for a resolution, but a reason not to search for a resolution is not a resolution.

 

[marcus]

...I think all this can be reduced to the following question:
Who is more clever, the physics equations, or the physicists who invented them?

If physicists are more clever, and equations merely represent a part of all things which they understand, then Mermin is right: Equations are nothing but a part of our description of our knowledge about the world, not the reality. If so, then there is no problem of now, no problem of interpretation of quantum mechanics, etc.

However, there are good reasons to believe that equations are more clever than the physicists who invented them...

...but NATURE is more than equations.

I don't see Nature as "more than", I see it as DISJOINT from the human description. Let's not confuse the description with the reality. This "clever equations" talk verges on superstition or mysticism. AFAIK math is an artificial human-invented language. The meaning of an equation can change as the variables get redefined. Equations have limited applicability and sometimes get discarded and replaced by improved equations which again have limited applicability and are subject to eventual improvement.

It certainly is, but that constatation alone cannot resolve any problem one might have with the equations. Perhaps it can give someone a reason not to search for a resolution, but a reason not to search for a resolution is not a resolution.

Demy, what can you mean by "search for a resolution"? One searches for improved understanding, a simpler better-fitting model, more precise reliable prediction. Do you imagine that there is some final "ontology"? A final equation that will tell us what Nature "IS"?

 

[marcus]

By pointing to the Gielen Wise paper, Strangerep raises the possibility that a next-generation REALIST description of the world could enjoy the feature that no reconstruction of spacetime is possible. The math description could describe an objective reality shared by all observers, but as a minor detail that description would contain no consensus continuum.

So? Big deal we would still be assuming an objective reality shared by all observers. But with the proviso that each observer has to construct an imagined 4D continuum for hermself. If that seems funny it is only because we have an engrained habit of presuming that any math description of objective common reality MUST include a 4D continuum. It is admittedly a widespread prejudice.

 

[marcus]

I see Strangerep already suggested "Maybe…" what I just now said. "The notion of a physically-real global spacetime manifold is so deeply entrenched." There's the rub. People naively confuse discarding that entrenched notion with "Solipsism". It's not. It's just a new line of mathematical investigation e.g. by such as Laurent Freidel and Derek Wise.

==Quote by atyy==
[...] If what [Mermin] is saying is just common sense (and it seems to be), why does he write it as if it's such a big deal?
===
Maybe because the notion of a physically-real global spacetime manifold is so deeply entrenched, yet easily questioned by pointing out that each of us only synthesize it based on received stimuli.

==Quote by atyy==
There is one part where I think I definitely disagree with him. Isn't the perception of "now" part of the "hard problem" of consciousness? Like is the "blue" I see the same as the "blue" you see?
===
It doesn't need to be "the same". The description of it only needs to be covariant (in a generalized sense -- my frequency perception spectrum must be consistently calibratable against yours, so that we can agree whether something is/isn't "blue").

Strangerep, This post reminds me of the C*algebra representation of reality used in several recent papers by Carlo Rovelli. There is no "spacetime" continuum but there is an algebra of observables, and there is a physical state which is a positive functional defined on that C* algebra.

Atyy,
Your previous reply prompted a followup thought that different observers also need to share a set of observables, presumably organized as a Lie algebra, so that they are working with a common set of types. But... then there are other issues: the algebras could appear different, while nevertheless being isomorphic.

In that particular treatment the existence of a physical state (not a hilbertspace vector, but a positive functional that assigns expectation values to observables) can be used to generate a global semigroup flow among the observables ("Tomita time"). So it has some parallels with what you mentioned.

 

[atyy]

marcus, I think you once said the one concern you had with the general boundary formalism is that it seems that one could not do quantum cosmology with it. But if one buys that one can at present apply the Copenhagen interpretation (by which I just mean shut-up-and-calculate) with a Heisenberg cut between the quantum and classical to eg. CMB aniostropies, then it seems that in principle the general boundary formalism might still allow us to answer in principle things like the resolution of the big bang singularity that I think you classify as part of quantum cosmology. Here my approach is yes there is an underlying reality (which is a yet unknown but useful model), and quantum mechanics is an "operational" theory in the sense that it requires two realities (classical and quantum) for us to use it, whereas a "reality based" model would have only one "reality". Would this work for you?

BTW, as a biologist, I can tell you everyone assumes their model is wrong, but hopefully it is useful. So it will be a big surprise to me if physicists are more naive than biologists. I'd imagine the difference between biologists and physicists is that physicists are more likely to suspect that any model will be incomplete in principle. In fact, I can claim it is textbook physics http://dao.mit.edu/~wen/book/preintro.pdf: "The physical theory that can be formulated cannot be the final ultimate theory. The classification that can be implemented cannot classify everything. The unformulatable ultimate theory does exist and governs the creation of the universe. The formulated theories describe the matter we see everyday."

BTW #2, I am still extremely befuddled by what Dittrich is doing with the general boundary formalism, even after seeing her Perimeter talk which you helpfully posted in some other thread ...